The repopulation of electronic states upon vibrational excitation of niobium carbide clusters

Abstract

We study the infrared (IR) resonant heating of neutral niobium carbide clusters probed through ultraviolet (UV) photoionization spectroscopy. The IR excitation not only changes the photoionization spectra for the photon energies above the ionization threshold, but also modulates ion yield for energies significantly below it. An attempt to describe the experimental spectra using either Fowler's theory or thermally populated vibrational states was not successful. However, the data can be fully modeled by vibrationally and rotationally broadened discrete electronic levels obtained from Density Functional Theory (DFT) calculations. The application of this method to spectra with different IR pulse energies not only yields information about the excited electronic states in the vicinity of the HOMO level, populated by manipulation of the vibrational coordinates of a cluster, but also can serve as an extra indicator for the cluster isomeric structure and corresponding DFT-calculated electronic levels.

Direct IR Spectroscopic Detection of a Low‐Lying Electronic State in a Metal Carbide Cluster

“…If the IR excitation frequency is fixed, and the UV photon energy is modified, a rather broad enhancement in gain is observed (Figure S4), regardless of whether electronic (458 cm −1 ) or vibrational (742 cm −1 ) bands are excited. Both of these UV spectra are similar to the curves presented earlier for clusters excited vibrationally ,. This points at a statistical redistribution of the energy over the vibrational states absorbed in the electronic excitation detected in this work.…”

“…Both of these UV spectra are similar to the curves presented earlier for clusters excited vibrationally. [5,6] This points at a statistical redistribution of the energy over the vibrational states absorbed in the electronic excitation detected in this work.…”

“…In a recent work, we have studied how the excitation of pure vibrational coordinates can be followed by a relaxation into electronically excited states through vibronic coupling. This way, low‐lying electronic states are revealed by the appearance of extra structure in the photoionization spectrum ,. The electronic states had energies of approx.…”

“…If not vibrational, what is then the origin of this band? For this, we note that in our earlier work on niobium carbide clusters, we found low‐lying electronic states at 0.35 eV, and that the TaC dimer anion exhibits an electronic state at 0.362 eV . It is thus not unreasonable to suspect that Ta 5 C 3 can also exhibit such low‐lying states.…”

“…This way, low-lying electronic states are revealed by the appearance of extra structure in the photoionization spectrum. [5,6] The electronic states had energies of approx. 0.3-0.5 eV above the ground state, and their detection efficiency was consistent with a thermal occupation, requiring the absorption of a large number of IR photons.…”